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1. The Field of the Invention
The present invention is related generally to optical filters for use in telecommunications components. More specifically, the present invention is related to optical filter devices that are passively thermally stabilized so as to maintain their optical properties in changing temperature conditions.
2. The Relevant Technology
Many modern optical systems, such as fiber optic systems used in the telecommunications industry, utilize optical narrow bandpass (NBP) filters to achieve wavelength purity. Such optical filters are utilized to multiplex and de-multiplex very narrowly spaced optical channels. Optical bandpass filters typically consist of either Fabry-Perot cavities or multilayer thin film dielectric layers of specific thicknesses that are deposited in the form of an optical coating on a suitable substrate. Both Fabry-Perot cavity filters and thin film optical filters typically change properties, such as transmission loss versus frequency, when subjected to variations in temperature. Some devices might be designed to operate in a temperature-stabilized environment; however, other devices might need to operate over a wide ambient temperature range, such as about xe2x88x925 to 70xc2x0 C. Optical components incorporating such filters thereby can exhibit undesirable performance variations. This is a particularly serious problem when the filters are used to perform single wavelength bandpass functions in high channel density optical fiber telecommunications systems such as dense wavelength division multiplexing (DWDM) systems.
The current trend is towards achieving a higher density of information transmission through optical fiber systems, which in turn requires a higher density of channels (and corresponding wavelengths) and also less spacing between channels. Channel spacings are thus being reduced from about 100 GHz to 25 GHz, and therefore when the optical properties of the bandpass filter change with a variation in temperature, the transmitted center wavelength can be shifted, resulting in non-transmitted signals or channels which overlap, causing errors and unreliability in the transmitted information. Such temperature-induced center wavelength shift is a limiting factor in how narrow the optical channels can be spaced and therefore how much information an optical fiber system can carry.
Various attempts have been made to circumvent the temperature stability problem associated with optical bandpass filters. For example, U.S. Pat. No. 5,375,181 by Miller et al. discloses a fiber Fabry-Perot bandpass filter composed of two partially transmissive reflectors separated by an optical cavity of a specific length, which transmits light of a single desired wavelength. Materials for the enclosure of the Fabry-Perot cavity are carefully chosen in cooperation with piezo-electric tuning to compensate for temperature-induced variations in the cavity length. Generally speaking, the filter materials expand with increasing temperature, thus extending the optical path lengths of the layers and shifting the frequency characteristics, such as the passband, to longer wavelengths, or xe2x80x9cdown spectrumxe2x80x9d. In some instances, the refractive index of a layer material might also be temperature dependent, thus the optical path length can vary according to both the thermal expansion/contraction and changes in the refractive index. However, incorporating piezoelectric tuning to compensate for temperature variations is relatively complex, and typically requires additional space for the electronic tuning circuitry, and thus may be undesirable for compact fiber optic communication components. Furthermore, the piezo-electric tuning approach requires an electric power source, such as a battery, which can deplete, or line power, which can be interrupted or fail, thus losing the desired tuning.
An approach to enhance the stability of optical filters in optical communication systems is described in U.S. Pat. No. 5,430,574 to Tehrani, in which piezoelectric transducers are used in conjunction with a Fabry-Perot cavity. The piezoelectric transducers are configured in such a way as to effect a change in the length of the cavity in accordance with the magnitude and polarity of control signals. Nevertheless, this approach is also relatively large and complicated to implement in optical communications networks, and is impractical for interference filter applications.
Another approach is described in a paper entitled xe2x80x9cTemperature Stability of Thin-Film Narrow-Bandpass Filters Produced by Ion-Assisted Depositionxe2x80x9d by Haruo Takashashi, Applied Optics, Vol. 34, No. 4, pp. 667-75 (February 1995). In the Takashashi paper, the problems associated with the shift in center wavelength of thin film bandpass filters with changes in temperature and humidity are described. A solution to the problem is proposed by forming the filter on a substrate that has a high coefficient of thermal expansion, which if made properly, can compensate for the changes in the optical properties of the bandpass filter that occur with changes in temperature. The refractive index changes that cause the CWL to increase with increasing temperature can be offset by utilizing a substrate made of a material with a high coefficient of thermal expansion that will stretch the thin film layers as the temperature rises and effectively decrease the physical thickness of the layers. High thermal expansion materials, however, have the disadvantage of being expensive specialized glasses that are difficult to handle and fabricate, as well as easily damaged or downgraded.
Accordingly, there is a need for improved optical filter stabilizing techniques that overcome the above difficulties.
A primary object of the present invention is to provide a device for encasing an optical bandpass filter that provides thermal stability to the optical properties of the filter.
Another object of the present invention is to provide methods for making optical filter devices that exhibit stability with respect to thermal and mechanical stress.
A further object of the present invention is to provide optical filter devices having thermally stabilized center wavelength bandpass characteristics.
To achieve the forgoing objects and in accordance with the invention as embodied and broadly described herein, an optical filter device is provided that includes a multilayer optical bandpass filter on a transparent substrate, with the transparent substrate being composed of a material having a first coefficient of thermal expansion. An encasement holds the transparent substrate such that the bandpass filter is exposed for transmission of optical signals there through. The encasement is composed of a material having a second coefficient of thermal expansion that is different than the first coefficient of thermal expansion. The encasement provides thermal stability to the optical properties of the bandpass filter by compensating for changes in the filter and filter substrate that occur with temperature variations.
Various methods can be utilized to assemble the filter devices of the invention. In one method, an encasement having a high thermal expansion coefficient is shrink fitted to an optical filter such that the substrate of the filter is placed under compression by the encasement in the operating temperature range. In other methods, the filter substrate is bonded above or below the desired operating range, such as with an adhesive, solder, or polymer resin, resulting in stress on the filter substrate that is compressive or tensile. In another embodiment, collet fingers hold the filter. A collet closer is adjusted to provide a selected amount of mechanical stress on the filter, thus providing a selected initial filter characteristic as well as improved thermal stability.
The optical filter devices of the invention are capable of exhibiting stability in their optical properties, such as center wavelength transmission, over a wide temperature range, and are relatively easy and inexpensive to fabricate. Furthermore, the assemblies do not require electronic circuitry or electric power to operate.
These and other aspects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.